Multi language site support

[Ze0nC]

Hi @JohnSundell

Thank you for creating Publish.

I wanted to make my site support multi-language, for example:

• example.com/en, for English index
• example.com/en/posts/article, for English article
• example.com/zh, for Chinese index
• example.com/zh/posts/article, for Chinese article

So I tried to:

• Add markdown files in different folders for each language
• Correlate markdown files in language variations of the same item.
• Output the html files in folders for each language.
• And keep behavior of standard Website unchanged.

You may check how to use it in documentation: generate-multi-language-site

I have applied it to my own site at: czj.io

The design is basically :

• Add var language: Language? to ContentProtocol, to assign each item/page/index a language.
• Add public protocol MultiLanguageWebsite: Website where ItemMetadata: MultiLanguageWebsiteItemMetadata
• Extend existing struct/protocol to handle multi-language website
• Added internal struct MultiLanguageContentManager to correlate items and links of different languages.

I had to override (mostly copy and paste) many functions to handle multi-language website while keeping original functions unchanged. This caused redundant in code.

If you agree, I want to try to merge the code of MultiLanguageWebsite into Website, to make codes shorter and clearer.

I understand that you are busy recently and may not have much time on checking this.
But I sincerely hope to hear from you and get feedback.

Cheers,
Zhijin

[JohnEstropia]

@JohnSundell Hi, this PR had aged a bit but I think multi-language support is still a desired feature.
What were your thoughts on how to approach localization?